A field experiment was conducted in 2001 at a paddy field of grey lowland soil (Typic Endoaquepts) in Faculty of Agriculture, Kochi University, Japan. The surface soil (0-15 cm) with characteristics of pH (H2O, 1:2.5 w/v), 5.40; EC, 0.072 dS m-1; total N, 1.17 g kg-1; total C, 12.7 g kg-1; exchangeable cations Ca, 2.87 cmolc kg-1; Mg, 0.38 cmolc kg-1; K, 0.26 cmolc kg-1; and CEC, 7.67 cmolc kg-1. Basal fertilizer was supplied at the rate of 100 kg N, 100 kg P2O5, and 100 kg K2O ha-1 by using a coated slow release fertilizer (E-80, 140 d-type, Kumiai, Co. Ltd.). Using NH4Cl, additional 20 kg N ha-1 was also applied with the slow release fertilizer. Silicate fertilizers were not incorporated in the field throughout the cultivation.
Four rice (Oryza sativa L.) cultivars Tadukan (indica), Nanjing 11 (indica), Nipponbare (japonica), and Arborio (javanica) were selected. Grain characteristics of the above mentioned cultivars are shown in Table 1. Two seedlings of 19 days old of each cultivar per hill were transplanted with spacing was 30 cm × 15 cm.Other cultural conditions were conventional ones. Plots of each cultivar were arranged following the randomized block design with two replications. Ten panicles of equal size were collected from different spots in each plot at heading, 10 days after heading (DAH), 20 DAH, 30 DAH and at maturity. All of the grains from panicles in each plot were mixed and composite samples were obtained. All of the spotted, distorted or abnormal seeds were discarded from the samples. In every composite sample, each grain was separated into a hull and a brown rice, and their dry weights were recorded. The contents of Si and other mineral elements in the samples were determined by using an inductively coupled plasma atomic spectrometry (ICP-AES, Shimadzu ICPS 1000IV) after fusion with lithium metaborate (Goto and Ninaki, 1991), unless otherwise stated. Potassium and Mg were determined by using atomic absorption spectrometer (Shimadzu AA-610S), and N was determined according to the Kjeldhal method.
Electron probe X-ray microanalysis (EPMA) of lemma
Inflorescence bract lemma and palea was dissected from the mature seeds at maturity. Lemma was then cut along the length. Prepared lemma was fixed to aluminum disks with adhesive and was coated with a carbon layer approximately 30nm thick using a vacuum evaporator (JEOL, JEE-400). Analysis was carried out using a wavelength dispersive type electron probe X-ray microanalyzer (JEOL, JXA-8600MX). The analysis of Si on the abaxial surface around the tip area of the hull were conducted at an accelerating voltage of 15 kV, a sample current of 5~10-8A, and with a beam diameter of 5 ƒÊm. The X-ray spectrometer was peaked for the first order silicon Kƒƒ line (0.7126 nm), using a thallium acid phthalate (TAP) diffracting crystal. The detection of Si was performed by X-ray distribution mapping with stage scanning mode. Morphological observation was performed using the secondary electron scanning microscope (SEM) image at a sample current of 2~10-9A.
Least significant difference (LSD) for the comparison of any pair of the data were analysed.